Investigations concerned with the cellular requirements involved in the in vitro immune responses of murine lymphoid cells constitute the broad area of interest. The participation of T-cells and their products on both the antibody response, and the cell-mediated response of murine spleen cells is being investigated by using cloned, antigen-specific T-cells and selected T-cell hybridomas. The effect of lymphokines and monokines on the activation and proliferation of purified B cells constitute another area of major interest intended to establish long term cultures of functional B-cells. Information derived from these studies is applied to similar investigations relevant to understanding the effect of age on the immune response.